Conventionally, electric devices (e.g., an electric shaver, a power toothbrush and a cellular phone) having secondary batteries as a power source are provided with various non-contact power supply systems to contactlessly charge the secondary batteries by using electromagnetic induction.
In a non-contact power supply system, when a primary coil of a power supply device is not magnetically coupled with a secondary coil of an electric device (load device) (a standby mode), an inverter circuit of the power supply device is intermittently driven to suppress power consumption, for example. When the primary coil is magnetically coupled with the secondary coil (a power supply mode), the inverter circuit is continuously driven to supply a large amount of power to the electric device. Further, when a metal foreign substrate is placed in the vicinity of the primary coil of the power supply device, the foreign substrate may be heated by induction heating. Accordingly, coils for signal transmission are provided in both the power supply device and the electric device. The inverter circuit of the power supply device is switched from an intermittent operation mode to a continuous operation mode only when a signal is transmitted from the electric device to the power supply device through the coils for signal transmission, thereby preventing the foreign substrate from being heated (see, e.g., Japanese Patent Application Publication Nos. H10-271713 and H8-80042).
However, in the above-mentioned conventional case, the inverter circuit of the power supply device is intermittently driven even in the standby mode in which power is not transmitted to the electric device. Accordingly, a relatively large amount of power is consumed even in the standby mode. Further, since a signal is transmitted from the electric device via the coil thereof for signal transmission, in the conventional case disclosed in Japanese Patent Application Publication No. H10-271713, an oscillation circuit for oscillating the coil for signal transmission and a second secondary coil which is magnetically coupled with the primary coil of the power supply device and supplied with power from the intermittently driven inverter circuit to supply power to the oscillation circuit are provided in the electric device. Accordingly, there is a problem in that space and cost are required to provide two secondary coils in the electric appliance.